As known in the art, the light extraction efficiency of light emitting solid state devices such as light emitting diodes (LEDs), Organic-LEDs (OLEDs), solid state lasers and other solid state light emitting devices is limited, to a large extent, by the out-coupling efficiency or extraction efficiency. The high refractive indices of the substrate and the light generating active layer (relative to the air) leads to total internal reflection (rather than emission) and as a result the wave-guiding of a significant portion of light generated in the active layer. As known in the art, for light to be extracted from the device the light must be within an escape cone which is defined by the critical angle for total internal reflection at the interface (e.g. substrate to air interface). Photons outside the escape cone experience repeated internal reflection and are eventually lost due to absorption. The higher the mismatch in the refractive index at the interface the smaller the escape cone. This is a known major challenge in the realization of practical solid state lighting devices.
The confinement of photons by different layers gives rise to different modes in light emitting devices. Photons entrapped in the active layer stack gives rise to active layer mode(s), photons confined by substrate to substrate mode(s), and photons that are extracted out of the device, to out-coupled mode(s).
Out-coupling efficiency has been improved by the opening of escape cones for each direction (lateral and vertical) by use of thick transparent substrates, shaping of the chips (e.g., LED chips) or by reducing wave-guiding through modification of various interfaces for the device. Interface modification induces photon randomization which changes the incident angle at each incidence, thereby providing multiple chances for photons to escape. Photon randomization has been achieved by simple interface roughening, such as by chemical etching, photochemical etching, electrochemical etching, or by having regular patterned structures at various interfaces, such as Bragg gratings, micro-rings, photonic crystals, microlenses, and micro-pyramids. These techniques can increase surface roughness, cause sub-surface damage, or introduce foreign material contamination into the near-surface layers. Furthermore, such approaches may be complicated, may only be applicable to specific (i.e. discrete) wavelengths, and/or may not be easily integrated into the manufacturing process.